1. Field of the Invention
The present invention relates to heaters and, more particularly, to a high-velocity, accurately responsive impingement heater which is able to direct constant-temperature air to a process.
2. Description of Related Art
Heaters are used throughout industry to provide heat to a specific location to carry out a particular process. The location may be a joint between two aircraft parts, and the process may be a curing process of a chemical resin used at the joint. The heat provided by the heater, typically by blown air, is required to carry out or to facilitate the curing process. Many curing processes require very precise temperatures in order to be carried out accurately and with the highest degree of quality and reliability.
This is particularly true when the curing process cures composite materials used on high-performance aircraft. The builders of such aircraft absolutely need to maintain strict control over every aspect of the manufacturing process, including the curing to the composite material, to ensure that the aircraft perform as specified and to ensure the safety of pilots and crew. Other processes may include curing ultra-low-reflectance coatings used in stealthy applications.
One of the difficulties in employing heaters is maintaining control of the temperature or, more specifically, maintaining a constant temperature at the location of the process. Heaters use a heating element to heat air which is then blown through a manifold positioned at the location of the process. The manifold is attached to the heater by an air conduit which allows the manifold to be positioned at the location.
Accordingly, the air passing through the heating element travels some distance before reaching the manifold and being blown out to the location to carry out the process. Thus, the temperature of the heated air immediately downstream of the heating element may not be the same as the temperature of the air being blown out of the manifold. This temperature difference is known as offset temperature. Further, many of the processes may be located at a position which does not allow the heater to be positioned closely so that a relatively long air conduit needs to be employed to position the manifold close to the location at which the process is to be carried out.
In conventional heaters, the heating element is located inside a heat-exchanger enclosure and typically is a coiled stainless-steel air line. This poses a number of problems. For example, the length, the diameter, and the wall thickness of the tubing are critical variables that affect the overall performance of the heater. Accordingly, any variation from heater to heater in any of these dimensions eliminates identical performance between heaters. Therefore, highly strict tolerances need to be maintained, thereby increasing production costs.
Another drawback of conventional heaters is that the heat generated by the heating element radiates both outwardly and inwardly from the tubular heating element. The air rushing through the tube can only remove heat from the inside diameter of the tube. Accordingly, efficiency decreases as the heater cannot make use of the outwardly radiated heat, which heat is wasted and lost through the outer diameter. In addition, many conventional heaters use fans to blow air across open heating elements which is inefficient.
Furthermore, conventional heaters require a highly trained operator to manually control the amount of heat being applied to the cure area. This use of specialized operators is inefficient and results in higher production costs. For example, the temperature controller on a number of conventional heaters is programmable. An engineer can determine offset temperatures required internally to achieve the desired cure temperatures at the process. The engineer can then program this information into the controller, thereby allowing a cure to be accomplished generally with no further operator intervention once the curing process is under way. This process is known as profiling. A drawback of profiling a cure is that it is time consuming and needs to be done every time the process changes for another cure, resulting in lower efficiency and higher costs. In addition, controllers are not sufficiently responsive to temperature fluctuations in the air at the location of the process, resulting in temperature lags and overshoots.
In view of the foregoing discussion, it is an object of the present invention to provide a high-velocity, accurately responsive impingement heater which mitigates and/or obviates the aforementioned drawbacks of conventional heaters.
It is another object of the invention to provide a high-velocity, accurately responsive impingement heater which provides heated air to a process location and maintains the heated air at a substantially constant process temperature.
These objects as well as other objects, features, and benefits of the present invention are achieved by providing a high-velocity, accurately responsive impingement heater which provides heated air to a process location for effecting a process. The heated air which is provided to the process location is maintained at a substantially constant temperature.
According to one aspect of the present invention, the heater includes an air line for conducting air from an inlet thereof to the process location. A heat exchanger, including a plurality of heating elements, is provided for heating air conducted through the air line. A driver, which is connected to the heat exchanger, receives power from a power supply and applies current to the heat exchanger. A controller is connected to the driver and receives a predetermined process temperature required for effecting the process. The controller also provides the driver with a control signal.
The heater further includes a pair of temperature sensors. A process temperature sensor is positioned at the process location and measures the temperature of the air provided to the process location. The process temperature sensor then provides a process temperature signal to the controller which is indicative of the air temperature at the process location. An internal temperature sensor is positioned immediately downstream from the heat exchanger and measures the temperature of the air at that location in the air line. The internal temperature sensor then provides an internal temperature signal to the controller which is indicative of the air temperature downstream from the heat exchanger.
Based upon the temperature signals from the temperature sensors, the controller provides the control signal to the driver for specifying the amount of heat required at the process location in order for the heated air to substantially equal the predetermined process temperature. The driver then applies a corresponding current to the heat exchanger in response to the control signal.
One of the advantages of the heater of the present invention is that profiling (as described above) is eliminated. Rather than an engineer determining the offset temperatures required for a particular process, the heater of the present invention simply requires an operator to enter the predetermined process temperature; offset temperatures do not need to be calculated, particularly from process to process. Accordingly, the heater according to the present invention is more efficient, economical, and easy to use than conventional heaters.
Another advantage of the heater of the present invention is that temperature lags and overshoots are substantially eliminated. As the internal temperature sensor is positioned immediately downstream from the heat exchanger, the air exiting the heat exchanger is immediately monitored. If any temperature fluctuation occurs, then the controller is able to quickly send a control signal indicative of such fluctuation to the drive to adjust the current applied to the heating elements. Accordingly, the heater is much more responsive to minor fluctuations in air temperature than conventional heaters, resulting in the substantial elimination of temperature lags and overshoots.
According to another aspect of the present invention, the heater may include a plurality of heater switches respectively connected to the plurality of heating elements of the heat exchanger. The heater switches enabling each of the heating elements to be manually energizable. This results in the current being applied by the driver to the energized heating elements of the heat exchanger to be inversely proportional to the number of heating elements manually energized. This manual switching on and off of heating elements allows an operator to select any number of heating elements to heat the air for effecting the process.
According to another aspect of the present, the heater may further include a three-phase transformer connected between the driver and the heat exchanger. The heating elements are preferably serially configured with the transformer so that a specific firing order of the heating elements is effected. In addition, a plurality of mercury-displaced switches may be respectively connected between the plurality of heating elements and the transformer.
The heater of the present invention employs a number of beneficial safety features. For example, according to a further aspect of the invention, the heater has a housing within which the heat exchanger, as well as the other components, is disposed. An emergency temperature sensor may be provided to monitor the temperature of ambient air within the housing. Accordingly, if the temperature within the housing reaches unsafe levels, the emergency temperature sensor may signal an alarm or high-temperature indicator to alert the operator.
The air line of the heater also has a number of advantages. For example, a manifold may be connected to the outlet of the air line for easy positioning near the process location and efficient distribution of heated air. Further, the air line is preferably configured to receive compressed air, rather than fan-driven air, for high-velocity, conduction to the process location, which aids in maintaining the temperature of the heated air while moving through the air line.
Other aspects, features, and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description with reference to the accompanying drawings.